1. Field of the Invention
The present invention relates to apparatus for sensing linear acceleration. More particularly, this invention pertains to an open loop a.c. accelerometer employing an optical fiber pickoff for generating an interferometric output measurement.
2. Background of the Prior Art
The measurement of flexure or deformation of an elastic body in response to acceleration or pressure comprises the operating principle of numerous acceleration and pressure sensors. While the amount of deformation or displacement can be determined interferometrically, mechanically, piezoelectrically or by changes in the capacitances or inductances between elements, all such systems are limited by their physical and operational peculiarities such as limited sensitivity, high cost, limited maximum deflection and operating environment sensitivities. Some errors due to operating conditions are fundamental, such as limited physical flexure capacity in response to acceleration that renders the desired output indistinguishable from signal components associated with noise sources. Other operating condition errors can result from changes in physical dimension, modulus of elasticity, index of refraction, etc. occasioned by temperature and pressure changes.
Interferometric strain measurements exhibit superior accuracy and resolution. When carried out by means of an optical fiber, interferometric systems include simple and rugged sensor devices with low power requirements, immunity to electromagnetic interference, and ready adaptability to remote sensing and high data rates. Interferometric measurements of acceleration and pressure employing an optical fiber medium can be accomplished through telemetric signal transmission of a multitude of sensors in a single fiber using time division multiplexing. The fibers are themselves relatively insensitive per unit length and not subject to errors due to ambient pressure, tension from acceleration, etc. Increasing leg length provides greater sensitivity.
A number of a.c. acceleration measurement devices have been developed that utilize disk-mounted spiral coils of optical fiber to produce a desirable push-pull effect. U.S. Pat. No. 4,959,539 discloses a hydrophone in which each surface of an elastic and circumferentially-supported disk is round with a flat spiral of optical fiber fixed thereto. Flexure of the disk shortens the optical path length of the spiral on one surface while lengthening it on the oppositely-facing surface. The disk may be mounted on a body so that an acoustic pressure differential to be measured exists across the disks with the spirals being connected for push-pull operation as two legs of a fiber optic interferometer. In one such hydrophone, a pair of the circumferentially-supported disks and associated optical fiber spirals are mounted on opposite sides of such a body with the outer spirals connected as one interferometer leg and the inner spirals as another leg so that the differences in the lengths of the legs due to acceleration-induced flexure of the disks are cancelled. Such a double disk arrangement offers twice the measurement sensitivity. U.S. Pat. No. 5,317,929 discloses a fiber optic accelerometer based upon the double-disk structure described above that includes a centrally-located mass which clamps the opposed flexible disks together. Again, flat spiral windings of optical fiber are fixed to the surfaces of the flexible disks, providing inputs to the legs of a measuring interferometer. By adding a central mass, the device obtains greater sensitivity to acceleration without any decrease in its ability to cancel or reject d.c. effects.
U.S. Pat. No. 5,369,485 teaches another fiber optic accelerometer that employs a cascaded arrangement of disks engaged at their central portions to a cylindrical post and, at their exterior portions, to a coaxial cylindrical mass. The axial dimension of the hollow cylindrical mass greatly exceeds its wall thickness. Again, optical fibers formed into flat coils are fixed to opposed sides of each of three disks that extend radially from the cylindrical post to the hollow cylindrical mass. The various coils are interconnected to eventually form the arms of an interferometer.
Each of the foregoing accelerometers derives optical signals as inputs to an interferometer from direct measurement of the deflections of flexible disks. As a consequence, the efficiency and strength of the signal derived is highly dependent upon the peculiarities of the reaction of the flexible disk to acceleration. Generally, one may expect the deformation of a flexible disk to assume a two-dimensional shape that includes an intermediate inflection point between its clamped center and circumference. Unfortunately, this does not coincide with the region of the overlying spiral coil that is most capable of changing optical path length in response to stressing. Thus, an inherent mismatch minimizes the amount of signal generated per unit of strain-induced acceleration in such prior art devices. Further, by mounting coils to opposed faces of the relatively-thin flexible disk, the difference in optical path lengths experienced in compression and tension are relatively minimal. Again, this leads to a relatively "weak" output at the interferometer in response to acceleration. Sensitivity at low levels of acceleration may, in fact cut off the useful range of the sensor.